Signal-generating apparatus



Feb. 29, 1944. H, M, SMITH SIGNAL GENERATING APPARATUS 2 Sheets-Sheet 1Original Filed Oct. 10, 1940 Feb. 29, 1944. H. M. SMITH I SIGNALGENERATING APPARATUS 2 Sheets-Sheet 2 Origirial Filed Oct; 10, 1940"Davao/1.0010110111700040 Inventor Henry MSmith,

y His Attorney Reisouetl eb. 29, 1944 SIGNAL-GENERATING APPARATUSHenry'M. Smith, Schenectady, N. Y, alllgnor to General New York ElectricCompany, a corporation of Original No. 2,335,705, dated November 30,1943,

Serial No. 360,610, October 10, 1940. Application for reissue January15, 1944, Serial No.

15- Claims.

The present invention relates to apparatus for generating an electricalsignal which is representative of a light image. It is especiallyconcemed with improvements in television camera tubes of the type inwhich signal-generation depends upon scanning a picture-charged surfacewith a moving electron beam.

In apparatus of the character specified, it is common to make use of aphotosensitive electrode having a large number of discrete elementalareas which tend to become positively charged by the loss oflight-released electrons. In a system of this kind, it is the functionof the scanning beam periodically to neutralize the charges accumulatedon the various photosensitive areas and by so doing to develops. varyingelectrical signal which is representative of the light values to whichsuch charges are due.

A difilculty encountered in the application of the system just describedconsists in the apparent impossibility of maintaining a suificientelectrical field in the vicinity of the photosensitive.

element to assure the removal from its surface of all or even of a largepercentage of the electrons released by the light to which the elementis exposed. Even if an adequate field is initially established by theuse of appropriate electrode structures, the necessary conditions ofoperation are such that the photosensitive element as a whole soonbecomes electrostatically charged in a. manner tending to reduce thefield to zero. For this reason, the sensitized element is obliged tofunction under conditions of low' efficiency as measured in terms of thesignal strength obtainable by exposing the element to a visible scene ofgiven light intensity and contrast.

It is an object of the present invention to provide an improvedsignal-generating system in which the pictureresponsive component may beoperated under conditions favoring the attainment of the bestperformance of which this component is capable.

A feature of the invention which is useful in the fulfillment of theforegoing object consists of an arrangement in which the scanned surfaceis provided by a porous insulating layer which is superimposed on thepicture-sensitive element and which normally charges to a potential atwhich it maintains a strong electrical field acting on the element. Inthe use of this arrangeamplification of the light image desired to bement, electrons released by light from the various areas of thephotosensitive element are accumulated-on adjacent areas of theinsulating layer and thus tend to produce localized variations in thecharge of this layer. As the various transmitted before it is caused toaffect the sensitive element with which the scanned insulating layer isassociated. This may be done, for example, by first converting the lightimage into an electron stream of corresponding cross-sectional patternand sufficiently accelerating the stream by electrical means to obtainamplification effects of the desired magnitude. Other and more specificmeans of amplification are described hereinafter.

The features which I desire to protect herein are pointed out withparticularity in the appended claims. The inventioh itself. togetherwith further objects and advantages thereof, may best be understood byreference to the following description taken in connection with thedraw-; ings in which Fig. 1 represents a signal-gener ating apparatussuitably embodying the invention; Fig. 2 is an enlarged fragmentary viewof one feature of the apparatus of Fig. 1; Fig. 3 represents a modifiedapplication of the invention: Fig. 3a is an enlarged detail of one partof Fig. 3; Fig. 4 shows a still further modification of the invention,and Fig. 4a is an enlarged detail of one part of Fig. 4.

Referring particularly to Fig. 1, there is shown a cathode ray tubewhich is designated as a whole by the numeral Ill and which includes anelongated shaft portion H and an enlarged bulbous-portion l2. and nearthe extremity thereof, there is provided an electron gun comprising thecom-bination of a cathode l4 and an apertured control electrode or gridI5. The cathode is preferably separately heated and includes for thatpurpose an internal resistance, heater indicated in dotted outline atII. The cathode is maintained at from several hundred to severalthousand volts below ground by means of a voltage source (indicated as abattery l9) which also serves to maintain-the electrode ii at a negativepotential with respect to the cathode.

Within the shaft portion H The electrons emitted by the cathode areaccelerated and focused into a concentrated beam by means of an anode 2|which is maintained at from several volts to several hundred volts aboveground by a voltage source 22 connected thereto. a coating of aconductive material such as aluminum or palladium applied to theinterior wall surface of the envelope and extending into the bulbousportion thereof. a

The end of the tube envelope which is remote from the electron gun isprovided with a generally planar surface 24 adapted to serve as a windowfor light radiations focused thereon, for example, by a lens systemindicated schematically at 25. The inner surface of the window 24 isprovided with a target (designated as a whole by the numeral 26) thenature of which will be more fully explained hereinafter. The variouselemental areas of this target are caused to be scanned by the electronbeam by means of two mutually perpendicular sets of magnetic deflectingcoils which are respectively indicated in diagrammatic fashion at 23 and23.

In accordance with the present invention, the target 23 comprises thecombination of a semitransparent conductive layer of photosensitivematerial having applied directly thereto a thin porous layer ofinsulating material. The structure of the target is best shown in Fig. 2which represents an enlarged fragmentary section of the envelope windowportion 24. According to the preferred form of the invention, theinterior surface of the window 24 is provided with an extremely thinconductive layer 3| consisting, for

example, of rhodium which has been sputtered or evaporated on thesurface of the window. It is the function of this layer to impartadditional transverse conductivity to the target so that the targetpotential may be fixed at a desired level. The layer 3i is connected toground through a resistor 32 which, as will be later explained, assistsin the development of a signal voltage. 7

On the layer 3| there is provided a conductive layer '33 ofphotosensitive material which is also of such thickness as to be ofsend-transparent character. -'l'.'his layer may comprise, for example,antimony which has been subjected to a caesium vapor or it may comprisethe combination of silver oxide and caesium. The layer 33 is covered inturn with a porous coating 34 of an insulating material such asmagnesium oxide, aluminum oxide, calcium fluoride or the like. Theporosity of this layer should be such thatit is ineffect constituted ofa large number of discrete elemental areas separated by small openingsor crevices.

With an arrangement such as that described, 'a light image falling onthe tube window 24 will release electrons from the photosensitive layer33 in a pattern determined by the variations of light and shadow in theimage. If the layer 33 is of sufllcient thinnessisay on the order of afew hundred angstrom units), the electrons thus released may escape fromthe side of the layer opposite the window 24. Under these circumstancesand for a reason to be shortly explained, the electrons escaping fromthe various points of the layer 33 will be received and retained by thecorrespondingly located elemental areas of the porous insulating layerII.

The explanation for the foregoing lies in the fact that the continuousscanning of the surface of the insulating layer 34 by the electron beamproduces secondary emission from the layer in The anode may comprise,for example,

such fashion as to cause its surface to become positvely charged. Thischar ing will continue until the insulating layer reaches approximatelythe potential of the electrode 2|, at which time the secondary emissionratio will drop to unity. Thereafter, in the absence of any disturbingeffect, the layer will tend to float stably at this potential.

It will be noted that with the circuit connections illustrated, thepotential of the electrode 2i is above the ground potential andconsequently above the potential at which the photosensitive layer 33 ismaintained. As a result of this circumstance, a positive field willexist in the vicinity of the surface of the photosensitive layer 33.With appropriate conditions of operation, this field may readily be madestrong enough to cause substantially all photoelectrons released fromthe photosensitive layer to escape from its inwardly directed surface.The electrons escaping from any element of the photosensitive layer willbe acquired by the positively charged surface of the nearest insulatingelement, thus reducing its positive charge by the amount of the electroncharge collected. The insulating layer as a whole will, therefore, becharacterized at any instant of time by the possession of electrostaticcharges which vary from point to point over its surface in accordancewith the variations of the light image projected through the window 2|.

As the various elements of the charged insulating layer are traversed bythe scanning beam, each element which differs in potential from theelectrode 2| may be expected to release secondary electrons in a ratiogreater than unity until the normal charge of the scanned element hasbeen restored. The total number of electrons rele ed by any givenelement during each scanning period will therefore be a direct functionof the extent to which its charge has departed from the normal valuebecause of electrons received by it from the photosensitive layer.

The variations in secondary electron current thus realized as thescanning beam progresses from point to point may be used to produce asignal voltage representative of the light image in an appropriatecircuit connected either with the electrode 2| (by which the secondaryelectrons are collected) or with the conductive layer 3|. Fig. 1 showsthe latter arrangement, and in connection with this figure, it isassumed that the voltage developed across the resistor 32 is applied asa signal voltage to an amplification system through which it is to eimpressed on a transmitting antenna or at er transmitting agency. Thisconnection has a particular advantage in that the current flowingthrough the resistor 32 necessarily contains a direct current componentwhich is representative of the average light intensity of the imageimpinging on the target surface. This current component may I be used inthe generation of a signal component useful to control the averagebrightness intensity of the picture being reproduced at the receivingstation.

The target construction described in the foregoing possesses a markedadvantage over previously used arrangements of which Iam aware in thatsubstantially the full photoemission of the photo-sensitive component ofthe target may be employed in signal production. This is a consequenceof the fact that a relatively strong electrical field is maintained atall'times in the vicinity of the photoelectric layer 33 due to thecharges formed on the insulating particles which compose the layer 34.As a result, transverse dispersion of the released photoelectrons (thatis, collection of photoelectrons' released from one area by adjacentareas) is substantially avoided.

An improvement in this respect of at leastseveral hundred percent may berealised over systems in which the photosensitive surface is scanneddirectly. v

"In addition to the advantage iustmentioned, the invention also avoidsobjectionable redistribution of secondary electrons produced by thescanning beam in that it prevents the collection of such electrons byareas adiacent to the region being scanned. This is due to the fact thatthe. photoelectrons deposited on the various elemental areas of theinsulating layer I4 cause them to assume a relatively negative potentialwhich is unfavorable to the collectionof electrons released from nearbysources.

Several procedures may be employed in the preparation of a target of thecharacter specified. One satisfactory procedure involves evaporating Irhodium on the interior surface of the window part 24 and covering therhodium layer with a semi-transparent evaporated layer of antimonybefore the parts of the tube envelope are assembled. Still withoutassembling the tube, the antimony is then covered with a porousinsulating deposit of magnesium oxide, aluminum oxide, or the like. Inthe case of magnesium oxide, the application may be made by burningmagnesium in an oxidizing atmosphere and permitting the resultantcolloidal dispersion of magnesium oxide to impinge on the surfacedesired to be coated. Alternatively, one may apply magnesium oxide,aluminum oxide or a similar substance by spreading or sifting the oxidein the form of a fine powder. (Adherence of the oxide particles to therhodium layer may be expected to occur as a result of Van de Waal andother attractive forces.) Thereafter, thetube is assembled and heated toa degassing temperature (preferably not above 300 C.) while connected toa vacuum pump.

After the preliminary evacuating steps, caesium is introduced bydistillation while the tube is maintained at a temperature of from 150to 250 C. With this procedure, the caesium penetrates the porousinsulating layer, becoming a1- loyed with the antimony and formingtherewith a combination of highly photosensitive charac ter. As a finalstep, the tube is sealed on the vacuum pump.

According to an alternative procedure, the tube is assembled immediatelyafter the rhodium layer is deposited. In this case, the antimony isintroduced and deposited on the rhodium after the tube is baked out.Next, caesium is distilled into the tube by a known method, and theinsulatin layer is finally deposited on the antimonycaesium layer. Thismay be done either by siiting an insulating po der on the surface of thelayer from a side tub ation adapted to be subsequently removed or byevaporating it from an auxiliary filament in the presence of an inertgas to assure a porous coating.

A still further method of preparing the photo- Bil sensitive targetinvolves sealing the parts of the uation of the tube, the silver oxideis photosensitined by distilling a small quantity of caesiumintothetubeandthenbaking'thetube at from 225 to 275 C. The sensitivityof the surface thus produced can be further enhanced, if desired, byevaporating a slight additional amount of silver on the same, followingthe evaporating procedure by a bakeout at from 150 to 200 C. Finally, aporous insulating layer may be deposited on the photosensitive surfaceby a sifting technique or one of the other techniques mentioned above.

A signal-generating electrode of the type described in the foregoing iswell adapted for use in a system which incorporates means for amplifyingor intensifying the light image before the image is caused to aflect thesignal-generating member. This may be done in one way by the arrangementshown in Fig. 3. In this figure. there is illustrated a dischargeenvelope to enclosing a cathode II, a control electrode 42, and afurther electrode ll, an appropriate potential relationship beingmaintained between these electrodes by means of voltage sources 44 and45. Magnetic coils 4'! and 48 are provided for the purpose of deflectingthe electron beam generated by the electrode system in order to producea scanning motion of the beam.

The end of the envelope 4. which is remote from the cathode Ii isprovided internally with a' photosensitive electrode 50 which mayconsist, for example, of antimony-caesium or of silver oxide which hasbeen sensitized by caesium. The electrode 5! is in spaced parallelrelation to another electrode or target comprising the combination of avery thin metal foil 52, consisting, for example, of aluminum of athickness of a few hundred to one or two thousand angstrom units and aporous insulating layer indicated at II (see Fig. 3a). The layer 53,which is exposed to the scanning action of the electron beam provided bythe electron gun structure, may be constituted of magnesium oxide oraluminum oxide deposited in the form of finely subdivided particles. Abattery serves to maintain the photosensitive element 5 at a relativelynegative potential with respect to the grounded foil 52 so thatelectrons released from the photosensitive surface by the impingement oia light image thereon are projected toward the surface of the foil. Thepotential diiiference between the elements 50 and 52 may readily be madesufficiently great so that a substantial acceleration of the electronstream generated from the surface of the element 50 is produced.

As they impinge upon the surface of the toil element 52, thephotoelectrons proceeding from the element will release secondaryelectrons.

If the foil 52 is very thin, as previously specified,

these electrons may penetrate the foil and escape therefrom at thesurface which is adjacent to the insulating layer 53. For the reasonsgiven in connection with the device of Fig. 1, this latter layer isnormally maintained at the potential of the electrode 43 by the actionof the scanning beam so that a strong local field exists at the surfaceof the foil 52. This field causes electrons released from the variousareas of the foil to be collected upon the correspondingly located arwsof the layer 53. The electronic charges thus produced are cyclicallyneutralized by the action of the scanning beam, such neutralizationbeing accompanied by the appearance of a signal voltage across aresistor 51 provided for that purpose in the grounding connection of thefoil 52.

number of photoelectrons themselves. ingly, there is obtained aneffective amplification of the light image prior to the occurrence ofany charging of the insulating layer 53. The fact that both the elements50 and 52 ar conductive and are of planar character facilitates themaintenance between them of a uniform electric field and assures thatthe cross section of the electron stream projected from the element 50shall maintain a pattern corresponding to the light image by which it isproduced. If desired, magnetic focusing means (not shown) may be used toenhance this eifect.

A still further means for obtaining image amplification is illustratedin Fig. 4. In this arrangement as in those previously described. thereis provided an evacuated discharge envelope 60 having therein a cathodeGI and a pair of appropriately biased cooperating electrodes 82 and 63.Defiecting coils 64, 65 are also provided.

As in the arrangement of Fig. 3, the end of the discharge envelope hason its inner wall surface a photosensitive layer 61. This is in spacedparallel relation to a signal-generating electrode or target whichincludes (see Fig. 4a) a thin non-translucent metallic layer 69, a layerof electron-responsive luminescent material ll sufflciently thin to beof semi-transparent character, a transparent supporting plate ll ofglass or mica, a thin conductive layer 16, a very thin, preferablysemi-transparent photosentitiv'e layer 12, and a porous insulating layer13. With this disposition of parts, photoelectrons released from theelement 61 are projected toward the metallic of the light image by whichthe el ent 81 is excited The opaque character of e metallic layer itprevents th occurrence of light radiation from the fluorescent layer inthe direction of the photosensitive element 61 and thus avoidsobjectionable secondary excitation of this element. All the lightdeveloped by the fluorescent material is, however, enabled to affect thephotosensitive layer 12 and to release photoelectrons from this layer.In accordance with the principles previously described herein, thereleased photoelectrons are collected on the positively charged surfaceof the insulating layer 13, being subsequently discharged by the actionof the scanning beam with the consequent generation of a signal voltage.(The signal voltage is caused to appear across a resistor ll connectedbetween the layer I6 and ground, just as described in connectionwithFigs. 1 and 2.)

It is assumed that in the operation of a system such as that of Fig. 4 asufficient potential is maintainedbetween the elements 61 and 69 (as bya battery 15) to assure that the light image produced by the fluorescentlayer 10 shall be of greater intensity than the primary image Accord-The arrangement just described has the; advantage that with propercorrelation of potentials the number of electrons released from the foil52 by the impingement of photoelectrons thereon may be considerablygreater than the thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the in: vention. I, therefore, aim in the appended claimsto cover all such equivalent variations as come within the true spiritand scope of the foregoing disclosure.

to which the excitation of the element 61 is due.

(From another standpoint this means that the electrons released from thephotosensitive element l2 exceed in number those released from theelement 61.) The amplification thus produced manifests itself usefullyin the production of a relatively greater charge on the various areas ofthe insulating layer 13 than might be the various areas in accordancewith the charges received thereon from the said element, and

means for collecting the said secondary electrons.

2. In a signal-generating apparatus, the combination which includes anelectrode element adapted to release electrons therefrom in a patterndetermined by image-bearing radiations impingingv on the element andhaving one side thereof exposed to the impingement of such radiations,the said element being sufficiently thin so that electrons released as aresult of radiations impinging on the said one side thereof may escapefromethe element at the opposite side thereof, a porous insulating layercontiguous with the last-mentioned side of said electrode element andproviding a large number of discrete exposed areas for receiving andretaining electrons escaping from corresponding areas of the element,

means for scanning the said exposed areas of the insulating layer with abeam of electrons to generate secondary electrons from the various areasin accordance with the charges received thereon,

' electrons.

3. In a cathode ray tube apparatus, a sheetlike photo-sensitive elementadapted to release electrons in a pattern determined by a light imageimpingingon the element, a porous layer of finely divided insulatingmaterial in direct contact with one surface of the element and providinga large number of discrete exposed areas for receiving electronsreleased from corrwponding areasof the element, means for scanning saidexposed areas with a beam of electrons to generate secondary electronsfrom the various areas in accordance with the charges received thereon,and means for collecting the said secondary electrons.

4. In a signal-generating apparatus, the structural combination whichincludes a partially transparent conductive metal layer, aphotosensitive layer superimposed on said conductive erate secondaryelectrons from the various exposed areas of the layer in accordance withthe charges received thereon from the photosensitive layer, and meansfor collecting the said secondary electrons.

5. In a signal-generating apparatus, the combination which includes atransparent supporting member a semi-transparent conductive layer ofrhodium on,said supporting member, a photosensitive layer superimposedon said rhodium layer and adapted to release electrons in a patterndetermined by a light image projected through the rhodium layer, aporous layer of iinely divided insulating material on saidphotosensitive layer and providing a large number of discrete exposedelemental areas for receiving electrons released from correspondingareas of the photosensitive layer, means for scanning said insulatinglayer with a beam of electrons to generate secondary electrons from thevarious exposed areas of the layer in accordance with the chargesreceived thereon from the photosensitive layer, and means for collectingthe said secondary electrons.

6. In a cathode ray tube apparatus, means providing a conductive surfaceadapted to release electrons in a pattern determined by image-bearingradiations impinging on the surface, a porous insulating layer on saidsurface and providing a large number of discrete exposed elemental areasfor receiving electrons released from corresponding areas of thesurface, means for scanning said insulating layer with a beam ofelectrons in order to generate secondary electrons from the variousexposed elemental areas of the layer in accordsaid surface, and anelectrode positioned in spaced relation with respect to said insulatinglayer for receiving secondary electrons from the layer, .said electrodebeing adapted to be maintained'at a positive potential with relation tosaid conductive surface.

7. In a signal-generating apparatus, an enclosing envelope, 9.photosensitive member at one end of the envelope adapted to releaseelectrons in a pattern determined by a light image impinging thereon andhavin one side thereof exposed to the impingement of such radiations,the member being sufllciently thin so that electron released as a resultof radiations impinging on the said one side thereof may escape from theelement at the opposite side thereof, a porous insulating layersuperimposed on the said opposite side of the photosensitive member andproviding a large number of discrete exposed elemental areas forreceiving electrons escaping from corresponding areas of thephotosensitive member, means including an electron beam source at theend of said envelope remote from said photo sensitive member forscanning the said insulating layer in order to generate secondaryelectrons from various exposed area of the layer in accordance with thecharges received thereon from the photosensitive member, and anelectrode positioned cooperatively with respect to said insulating layerfor receiving secondary electrons lecting the said secondary electrons.

thinness and being maintained at such potential that electrom impingingon the said one side are eirective to release electrons from the otherside thereof in a ratio greater than unity, a porous layer of finelydivided insulating material applied to the last-named side 0! the targetand providing a large number of discrete exposed areas for receivingelectrons released from corresponding areas of the target, means forscanning the insulating layer with a beam of electrons in order togenerate secondary electrons from the various exposed areas or the layerin accordance with the electronic charges received thereon from the saidtarget, and means for col- 9. In combination, means for producing anelectron stream having a cross-sectional pattern which is-representativeof a light image, an electron permeable metallic foil exposed to theimpingement of said stream, a porous layer of finely divided insulatingmaterial on said foil and providing a large number of discrete elementalareas for receiving secondary electrons released from correspondingareas of the toil by the impingement oi the said electron streamthereon, circult means for maintaining the toil at such a potential thatthe number of secondary electrons emitted therefrom exceeds the numberof electrons in the said stream, means for scanning the said insulatinglayer with an electron beam in order to generate secondary electronsfrom the various elemental areas thereof in'.accordance with thesecondary electronic charges received by such areas from the foil, andmeans for collecting the said secondary electrons generated from saidinsulating layer.

10. In combination, means for producing an electron stream having across-sectional pattern which is representative of a light image, meansproviding a layer 01' electron-sensitive luminescent material exposed tothe impingement-of said stream, a layer of photosensitive material inlight-receiving relation with respect to said luminescent layer, a layerof porous insulating material superimposed directly on saidphotosensitive material and providing a large number of discrete exposedelemental areas for receiving and retaining electrons released fromcorresponding areas of the photosensitive material by light projectedthereon, means for scanning the said indiscrete uniformly distributedand exposed areas for receiving and retaining electrons released fromcorresponding areas of the element, a collector electrode in spacedrelation with said layer,

and means for scanning the exposed areas of the insulating layer with abeam of electrons thereby charging said areas positively and forreleasing secondary electrons from the various areas in accordance withthe charges produced thereon by said element whereby signal indicationsmay be derived from either said collector electrode by variations in thesecondary electron emission current due to variations in the potentialdiii'erence between said layer and said collector or from said elementby virtue of the electrostatic coupling between said areas and saidelement.

12. In a cathode ray tube apparatus, a sheetlike photo-sensitive elementadapted to release electrons therefromin a pattern determined by a lightimage impinging on the element. a homogeneously distributed porous layerof finely divided insulating material in direct contact with one surfaceof the element and providing a large number of discrete uniformlydistributedand exposed areas for receiving electrons released fromcorresponding areas of the element, collector means in spaced relationfrom said layer, and

means for scanning the exposed areas with a beam of electrons togenerate secondary electrons mined by a light image projected throu hthe conductive layer. a homogeneously distributed porous layer of finelydivided insulating material on said photo-sensitive layer and providinga large number of discrete elemental areas for receiving electronsreleased from opposite areas of the photo-sensitive layer. collectorelectrode means, means for scanning said porous layer with a beam ofelectrons for maintaining said porous layer at substantially collectorelectrode potential for small light-levels or zero light intensity byvirtue of the secondary-electron emisfrom the various areas therebymaintaining said areas at substantially collector means potential at lowlevels of light intensity impinging on said element and for producingvariations in the secondary-electrcn emission current transmittedbetween the areas of said layer and said collector means upon variationsoi the potential 01' the areas occasioned by the photo-electron currentbetween said element and said areas.

13. In a cathode ray type tube apparatus for producing variations in anelectrical quantity re-' sponsive to a light image and which comprisession current flow from the various exposed areas, the signal beingobtained by virtue of the difference in current flow between the variousexposed areas incident to the change in area potentials V occasioned bythe photo-electron current bemeen said porous layer and saidphoto-sensitive 15. In a signal-generating apparatus, an element adaptedto release electrons therefrom in a pattern determined by image-bearingradiations impinging on the element, a uniformly distributed porousinsulating layer in direct contact with the surface of-said element andproviding a large number of discrete and exposed a. phcto-emisslveelement and an energy storage porous layer in contact therewith, themethod or operation which comprises scanning said porous layer with anelectron beam and maintaining the potential of said layer at collectorelectrode potential by virtue of the secondary-electron emission betweensaid layer and said beam, causing variations in the potential ofdiscrete areas of said layer by virtue of photo-emissive electroncurrent flow between said element and said layer and utilizing thecurrent flow in said collector electrode for furnishing an indication ofthe pattern of light intensity.

. 14. In a signal-generating apparatus, the combination including apartially transparent conductive metallic layer, a photo-sensitive layersuperimposed on said conductive layer and adapted to release electronsin a pattern deterthe scanning operation of areas for receiving andretaining the electrons released from corresponding areas of theelement, a collector electrode in spaced relation with said layer, andmeans for scanning the exposed areas of the insulating layer with a beamof electron thereby charging said areas positively due to thesecondary-electron current flow from said areas to said collectorelectrode, the magnitude of the current flow incident to the impingementof the electron beam thereon being a function or the charge thereof dueto the photoelectrcn current transmitted from said element to the areas,and said discrete areas serving to prevent interchange of chargethereamong due to cross fields between such areas of the insulatinglayer thereby maintaining sulllcient negative differential chargesbetween said areas to produce distinct 8 1 variations when collectedduring said beam.

HENRY M. SMI'I'H.

